1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program such as a wireless LAN (Local Area Network) for intercommunication between a plurality of wireless stations. More specifically, the present invention concerns a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for managing a wireless network by providing direct communication (random access) between communication stations.
Further more specifically, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program so that communication stations can be networked in an autonomous and distributed manner without providing relationship between a controlling station and a controlled station. In particular, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program so that respective communication stations can autonomously operate in synchronization with each other by maintaining an equal frame interval.
2. Description of Related Art
Canonical standards concerning wireless networks can include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), HiperLAN/2 (e.g., see non-patent document 2 or 3), IEEE802.15.3, and Bluetooth communication, for example. The IEEE802.11 has enhanced standards such as IEEE802.11a (e.g., see non-patent document 4), b, and g depending on differences of wireless communication systems and frequency bands.
According to a general method of constructing a local area network using the wireless technology, there is provided one apparatus called an “access point” or a “coordinator” functioning as a control station inside the area. The network is formed under overall control of the control station. The following access control method based on the band reservation is used for a wireless network where access points are disposed. When a given communication apparatus transmits information, a band needed for the information transmission is reserved for access points. Transmission paths are used so as to avoid collision with the information transmission in the other communication apparatuses.
There may be a case where asynchronous communication is performed between transmitting and receiving communication apparatuses in a wireless communication system containing access points. In this case, the wireless communication always needs to be routed through access points. Consequently, the transmission path performance decreases by half.
Another proposed method of constructing a wireless network is the “ad-hoc communication” that allows terminals to directly and a synchronously perform wireless communication. An ad-hoc wireless communication system has no central control station. Accordingly, this system is suited for constructing a home network comprising domestic electric appliances. The ad-hoc network has several features. For example, if one apparatus fails or is powered off, the routing is automatically changed, making the network strong against crash. Since a packet is hopped more than once between mobile stations, data can be transferred to a relatively distant destination by maintaining a high data rate. There are known various development examples about the ad-hoc system (e.g., see non-patent document 5).
In an IEEE802.11 wireless LAN system, for example, the IEEE802.11 networking comprises a BSS (Basic Service Set) and an IBSS (Independent BSS). The BSS is defined by “infra mode” in which a master such as an access point (control station). The IBSS is defined by “ad-hoc” mode comprising only a plurality of MTs (Mobile Terminals: mobile stations).
The infra mode BSS requires an access point that is responsible for coordination in the wireless communication system. That is, the access point transmits a control signal called a beacon at a proper time interval. When a mobile station can receive the beacon, the mobile station recognizes that the access point is available nearby. Further, the mobile station establishes connection with the access point to define a reachable range of radio waves around the station itself as the BSS, thus constituting a so-called “cell” in terms of the cellular system.
A mobile station near the access point receives a beacon. The mobile station can estimate the next beacon transmission time by decoding an internal TBTT (beacon transmission timing) field. Depending on cases (when no reception is needed), the mobile station may turn off the receiver and enable a sleep state (to be described later) until the next TBTT or several occurrences of TBTT ahead.
On the other hand, when an IBSS in the ad-hoc mode is to be used, a plurality of mobile stations starts negotiations with each other and then autonomously defines the IBSS. When the IBSS is defined, a group of mobile stations completes negotiations and then defines the TBTT at a specified interval. Each mobile station references a clock inside itself to recognize that the TBTT is reached after a random delay time, the mobile station may recognize that no other mobile stations transmit a beacon. In this case, the mobile station transmits a beacon. Also when the IBSS in the ad-hoc mode is to be used, the mobile station may turn off the transmitter-receiver to enable the sleep state as needed.
The IEEE802.11 networking also has a power save mode. A communication station in the power save mode receives only one beacon in several times and performs no transmission or reception in the other periods of time to minimize the power consumption. The access point stores a packet destined to a communication station in the power save mode and notifies the communication station that there is the packet buffered by the beacon. By receiving the beacon, the terminal is notified that there is the packet destined to the terminal itself. The terminal then notifies the access point that the terminal will receive the packet to receive the traffic.
[Non-patent document 1] International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent document 2] ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
[Non-patent document 4] Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) specifications: High-speed Physical Layer in the 5 GHZ Band
[Non-patent document 5] C. K. Tho, “Ad Hoc Mobile Wireless Network” (Prentice Hall PTR)
[Non-patent document 6] IEEE Std 802.11, 1999 Edition (Wireless LAN Medium Access Control and Physical Layer Specification 11.1 Synchronization P123-P128)
As mentioned above, in the autonomous distributed wireless communication system that does not necessarily require the control station, respective communication stations periodically notify beacon information on a channel. In this manner, each communication station notifies its presence and the network configuration to the other neighboring communication stations (i.e., in a communicable range). The communication station transmits a beacon at the beginning of a transmission frame cycle. Accordingly, a transmission frame cycle is defined by a beacon interval. The respective communication stations scan the channel only during a period equivalent to the transmission frame cycle to find beacon signals transmitted from the peripheral stations. The communication station decodes the information described in the beacon to be able to identify the network configuration (or enter the network).
In this wireless communication system, each communication station needs to ensure time synchronization with peripheral stations and periodically notify and manage the beacon information as mentioned above. For example, each communication station sets a prioritized utilization period within a frame cycle. The time synchronization is very important for such access system based on the time synchronization.
Many conventional wireless communication systems requiring time synchronization use an intermediate control station so that respective communication stations can ensure the time synchronization with the control station. This makes it possible to ensure the time synchronization between the respective communication stations existing in the same network.
On the contrary, the autonomous distributed wireless communication system has no relationship between the control station and a controlled station. Since there is no communication station working as the control station, it is impossible to use the conventional method of ensuring the time synchronization.
The infrastructure communication uses the access point as an intermediate to implement the communication. By comparison, the ad-hoc communication requires a large process amount in each communication station. Accordingly, increasing processes is unpreferable for the ad-hoc communication. For this reason, the autonomous distributed communication system requires a technology that can ensure the time synchronization with communication stations using a relatively simple process.
For example, as described in Non-patent document 2, IEEE802.11 adopts the clock synchronization method that notifies time stamp information describing the beacon transmission station's transmission time by including that information in a beacon. In this case, a communication station receiving the beacon adjusts the station's time to the same value as the time stamp in the infra mode. In the ad-hoc mode, the communication station adjusts the station's time to the same value as the time stamp when the time stamp value is later than the station's time. In this manner, the communication station always measures a clock error to adjust the time for synchronization.
Acquisition of the clock synchronization needs to not only correct misalignment of the time used as a basis for communication stations, but also synchronize the time progress rate (i.e., clock cycle). However, the clock synchronization method using the time stamp information can only perform the former, i.e., the time synchronization.
When the clock accuracy is insufficient, let us consider a case where a communication station in the power save mode keeps the sleep state for a long time. When the communication station receives no beacon too long, the time misalignment between communication stations exceeds a tolerance. Consequently, the clock accuracy restricts a period that permits the sleep state to continue in the power save mode.